Baby not gaining weight

Cassie • 💍10/1/17 💜6/16/11 💙6/2/19

So my son was born 6lbs 7oz and dropped to 6lbs 3oz in the hospital. At his 2 week appt he was still only 6lbs 3oz. They had me supplement with formula every other feeding and the next two weeks he gained 5 oz each week. I cut back on the supplementing because he seemed satisfied after breastfeeding. At his 2 month appt he was only 8lbs 1oz. Also, he spits up a lot and is on Zantac for reflux. Doctor doesn’t feel like he’s gaining enough and wants me to supplement more. Has anyone else gone through this?